Morphogenetic potentials of hydra tissue involved in head or foot formation were examined in a standard wild-type strain (105) and a mutant strain (reg-16) which has a very low head regenerative but a nearly normal foot regenerative capacity (T. Sugiyama and T. Fujisawa, 1977, J. Embryol. Exp. Morphol. 42, 65-77). Hydra tissue has two types of morphogenetic potentials to control head formation: the potential to form head structure (head-activation potential) and the potential to inhibit head formation (head-inhibition potential). It also has two types of morphogenetic potentials to control foot formation: foot-activation and foot-inhibition potentials. A lateral tissue grafting procedure (G. Webster and L. Wolpert, 1966, J. Embryol. Exp. Morphol. 16, 91-104), was used to examine and compare the relative levels of these potentials in the normal and the mutant strains. The potential levels were examined along the body axis of the intact animals and also in the regenerating animals after head removal. The results obtained show that the potentials involved in head formation are highly abnormal, whereas the potentials involved in foot formation are apparently normal in the mutant strain (reg-16). This suggests that the abnormal potentials are related in some way to, and may be responsible for, the reduced head regenerative capacity in the mutant strain reg-16.